Superconducting coils are used for the generation of strong, homogenous and temporally stable magnetic fields which are operated in continuous current mode. Homogenous magnetic fields with magnetic flux densities between 0.2 T and 20 T are, for example, required for Nuclear Magnetic Resonance spectroscopy (NMR spectroscopy) and for magnetic resonance imaging. These magnets are typically charged via an external circuit and then separated from the external power source as an almost loss-free current flow takes place via the superconducting coil in the resulting continuous current mode. The resulting strong magnetic field is particularly stable temporally as it is not influenced by the noise contributions of an external circuit.
When using known superconducting coil windings, one or more superconducting wires are wound on supporting bodies, various wire sections being brought into contact with each other via wire connections with the smallest possible ohmic resistance or via superconducting connections. For known low-temperature superconductors such as NbTi and Nb3Sn with transition temperatures below 23 K, there are technologies for making superconducting contacts for the linking-up of wire sections and for the connection of windings with a so-called superconducting continuous power switch. The known superconducting continuous power switches are each part of the circuit of the coil and are put into a resistive conducting state for the supply of an external current by means of heating. After switching off the heating and cooling to the operating temperature, this part of the coil also becomes superconductive again.
High-temperature superconductors or High-Tc-Superconductors (HTS) are superconducting materials with a transition temperature of more than 25 K and in the case of some material classes, for example, cuprate superconductors, of more than 77 K, for which the operating temperature can be reached by cooling with other cryogenic materials as liquid helium. HTS materials are particularly attractive for the production of magnetic coils for NMR spectroscopy and magnetic resonance imaging as some materials have high upper critical magnetic fields of more than 20 T. As a result of the higher critical magnetic fields, in principle HTS materials are better suited to the generation of high magnetic fields of, for example, more than 3 T or even more than 10 T than low-temperature superconductors.
A problem with the production of HTS magnetic coils is the lack of suitable technologies for the production of superconducting HTS connections, in particular, for second-generation HTS, so-called 2G HTS. 2G HTS wires are typically available in the form of flat coated conductors. If ohmic contacts are inserted between the superconducting coated conductors, the losses in the coil can no longer be ignored and the magnetic field generated declines markedly within a period of a few hours or days.
DE 10 2010 042 598 A1 specifies a superconducting MR magnet arrangement which has a superconducting coated conductor which is provided with a slit between the two ends in a longitudinal direction, the superconducting coated conductor thus forming a closed loop surrounding the slit. In the magnet arrangement the superconducting coated conductor is wound into at least one double coil comprising two coil sections which are twisted against each other in such a way that they generate a predetermined magnetic field course in a measurement volume. The subsequent introduction of superconducting connections is unnecessary for such a coated conductor with doubly connected topology. A superconducting continuous power switch can be formed in turn by a heatable local subsection of the conductor loop, this subsection being surrounded by two contacts for connection to an external feed current for feeding a current into the coil winding.
A disadvantage of such a known coil device is that the heatable area of the coated conductor constitutes a weak point at which the coated conductor is particularly susceptible to delamination and other damage. If, for example, damage occurs when fastening the heating, the entire coated conductor is destroyed thereby because no subsequent contacts can be introduced for repair due to the principle of the continuous slotted conductor loop. A further disadvantage is that for the heating additional current supplies are required for connection to a heating circuit, which must be routed from a warm external environment into the environment of the superconductor at a cryogenic temperature. As a result of these additional current supplies, additional paths for thermal losses are created, thus making the cooling of the superconducting coated conductor to its operating temperature more difficult. Additional disadvantages result from relatively slow switching behavior and the relatively long conductor areas required for thermal switching.